In a large gas engine including a main combustion chamber and an auxiliary combustion chamber, a fuel gas in the auxiliary combustion chamber, the volume of which is small, is combusted at a theoretical air-fuel ratio. By using the energy of the combustion, a low-concentration lean fuel in the main combustion chamber is combusted. This method realizes lean combustion and makes it possible to perform highly efficient operation in which NOx production is suppressed.
In an engine in which an auxiliary combustion chamber is disposed at the central part of a main combustion chamber, combustion is performed in such a manner that a flame is propagated from the central part to the peripheral part in the main combustion chamber.
If the propagating flame does not reach the peripheral part, the fuel at the peripheral part is not burnt and guided by an exhaust gas pipe together with an exhaust gas to be released to the external air. As a result, engine efficiency degrades.
Moreover, when pressure propagates to the peripheral part due to combustion at the central part, if unburnt gas at the peripheral part is compressed by the pressure and ignited before the flame propagates to the peripheral part, causing knocking, then there is a risk that the pressure and temperature in the combustion chamber rapidly increase, which may cause damage to the engine.
FIG. 5 is a cross-sectional view showing an example of a combustion chamber in a conventional large gas engine.
In a large gas engine having a structure in which nozzle holes of an auxiliary combustion chamber are exposed at the central portion of the ceiling of a main combustion chamber as shown in FIG. 5, a combustion gas produced in the auxiliary combustion chamber is injected into the main combustion chamber as flame jets through the nozzle holes. By the flame jets, a lean fuel in the main combustion chamber is combusted.
A piston cavity is formed at the center of the top surface of a piston. The piston cavity forms a large part of the volume of the combustion chamber when the piston is positioned at the top dead center. By forming the piston cavity, the volume of the main combustion chamber is increased. A side wall of the piston cavity forms a surface which the flame jets hit perpendicularly. A flat surface formed outside of the piston cavity and the ceiling of a cylinder form a top clearance.
In the conventional large gas engine shown in FIG. 5, the flame jets ejected from the nozzle holes of the auxiliary combustion chamber collide with the side wall of the piston cavity. Accordingly, a fuel gas in the piston cavity is combusted. However, a flame that occurs when the fuel gas is combusted by the flame jets takes some time to propagate to the top clearance. Accordingly, combustion in the top clearance is delayed from the combustion in the piston cavity, and thereby knocking occurs. There is also a case where the fuel gas in the top clearance is compressed due to the combustion in the piston cavity and thereby ignited before the flame propagates to the top clearance, causing knocking. This may cause damage to the engine.
In this respect, there is a method of preventing knocking in the following manner: the volume of the peripheral part of a combustion chamber is designed to be small and thereby the amount of fuel at the peripheral part is reduced, so that the amount of unburnt gas released to the atmosphere is reduced; and wall cooling is performed to suppress ignition.
However, in the case where the volume of the peripheral part of the combustion chamber is designed to be small, it is necessary to design the volume of the central part of the main combustion chamber to be great in order to maintain a certain compression ratio. Moreover, if the volume of the peripheral part is designed to be small, there is a case where it becomes necessary to form a valve recess in order to prevent interference between the piston and supply and exhaust valves. However, smooth flame propagation to the valve recess is more difficult.
Patent Literature 1 discloses a divided-chamber engine, in which the shape of a nozzle hole of an auxiliary combustion chamber is optimized, such that a flame jet directly reaches a valve recess at a peripheral part. In this manner, flame propagation to the valve recess is facilitated. However, in the method disclosed in Patent Literature 1, it is difficult for a fuel gas to flow into the peripheral part, and the concentration of the fuel gas at the peripheral part is low, accordingly. Moreover, since a wall cooling effect is exerted, even if the flame jet from the auxiliary combustion chamber reaches the peripheral part, combustion does not easily occur and unburnt gas remains at the peripheral part. Furthermore, since the fuel gas at the peripheral part is combusted at last, there is a high possibility that knocking occurs.